Self diagnostic switch circuit

Abstract

A switch circuit includes an input section which is connected to a detector section by a pair of leads. The input section draws a minimum current at all times through these leads and it draws more current when a shutter is operated to expose a photo sensor to a light source. The detector section senses the current level and thereby provides an indication of shutter operation. The detector section also indicates a fault condition when the minimum current is not drawn or excessive current is drawn.

I claim : 
     
       1. A switch circuit which comprises: a power supply connected to provide current through leads;   means coupled to the leads for detecting the amount of current delivered by the power supply;   a series circuit connected across said leads and including a light sensor connected in series with a first current drain, the series circuit being operable when light is received at the light sensor to conduct current;   a second current drain connected across the leads and in parallel with the series circuit, the second current drain being operable to conduct a preselected minimum amount of current;   light emitting means connected to conduct current which is supplied by the power supply through the leads and being positioned to emit light on the light sensor;   shutter means positioned to block the light applied to the light sensor when the shutter means is in one operating position and to allow the light to be applied to the light sensor when the shutter means is in a second operating position;   wherein the means for detecting the amount of current delivered by the power supply includes:   first means for providing an indication that the current has dropped below a level less than that conducted by the second current drain;   second means for providing an indication that the current has risen above a level which is greater than the sum of the currents conducted by the first and second current drains; and   third means for providing an indication that the current has reached a level which is greater than that conducted by the second current drain, but less than the sum of the currents conducted by the first and second current drains.   
     
     
       2. The switch circuit as recited in claim 1 in which the means for detecting the amount of current delivered by the power supply includes gate means coupled to the first and second means to provide a fault indication signal when either of the first or second means provides an indication. 
     
     
       3. The switch circuit as recited in claim 1 in which the light emitting means is connected to the power supply leads to conduct the current supplied to said first and second current drains.